Mobile assisted relay selection in a telecommunications system

ABSTRACT

A mobile station is operated in a telecommunications system that includes a base station and one or more relay stations. The mobile station communicates directly with the base station via a first channel. To facilitate selection of a suitable relay station, the mobile station broadcasts a burst for receipt by a plurality of relay stations. Relay stations each measure a quality of the received burst and communicate this information to the base station, which uses the information to select a most suitable one of the relay stations for use in carrying on indirect communications between the base station and the mobile station.

BACKGROUND

The invention relates to mobile telecommunication systems, moreparticularly to mobile telecommunication systems in which mobileterminals communicate with fixed base stations by means of relaystations, and even more particularly to selection of a most suitable oneof a number of relay stations for use in communications between a mobileterminal and a base station. Other aspects relate to architectures formobile telecommunication systems that employ base stations and relaystations.

Cellular communication systems evolving from third-generation (3G) andfourth-generation (4G) systems may provide data rates well exceeding 25Mb/s, perhaps even exceeding 100 Mb/s. For a receiver in such a systemto be able to operate at an acceptable error rate, a minimum signal tonoise ratio (SNR) per bit (Eb/N0) is required. What the minimum Eb/N0 isdepends on the modulation and coding schemes used and is on the order of5 dB for coded Quadrature Phase Shift Keying (QPSK) systems, but can beas high as 25 dB for complex modulation schemes like 64-QuadratureAmplitude Modulation (64-QAM). The energy per bit, Eb, is proportionalto the received power P_(rx), and inversely proportional to the datarate R in accordance with:

Eb=P _(rx) −R dB Joule

The received power P_(rx) is related to the transmit power P_(tx), theantenna gain G, and the propagation loss PL caused by distance and otherradio propagation effects in accordance with:

P _(rx) =P _(tx) +G−PL dB Joule

The higher the data rate R, the more the received power P_(rx) has toincrease to keep the energy received per bit Eb at an acceptable minimumlevel to ensure an acceptable bit error rate (BER). Since the outputpower P_(tx) of the transmitters is limited (especially in mobilestations due to battery limitations and limited heat-sink capabilities),the increased received power must be achieved by increasing the gain ordecreasing the path loss. The gain can be increased using directionalantennas. Smart, adaptive antennas are required since the mobilestation's position changes with respect to the base station.Alternatively, or in addition, extra base stations are deployed toreduce the distance between the mobile terminal and the nearest basestation.

Cell splitting is a known method for decreasing the distance between thebase station (BS) and the mobile station (MS). The idea with cellsplitting is to have more than one cell site serve a particulargeographical area. Each cell site can thereby cover a smaller area, andaccordingly use lower power levels which in turn creates the opportunityto reuse frequencies more times within a given system. Until now,geographical area operators have applied cell splitting to increasecapacity in their systems. In the next generation cellular systems, cellsplitting will be applied to reduce the mobile station-base stationdistance.

Cell splitting is costly for the system operators. It requiresadditional base station equipment as well as real estate at which toplace the base stations. Therefore, the use of relay stations (RSs) isbeing considered as a substitute for the additional base stationequipment. Instead of a fully functional base station, the relay stationmerely receives and forwards the signals between the base station andthe mobile station. FIG. 1 illustrates an exemplary arrangementcomprising a cell 125 having a base station 120, a mobile station 140,and several relay stations 130, . . . , 135. Since the relativepositions of the relay stations with respect to the base station arefixed, simple directional antennas can be used between the base station120 and each of the relay stations 130, . . . , 135. Each of the relaystations 130, . . . , 135 and base station 120 can therefore communicatewith very high data rates without the need for excessive transmit power.Because the distance between the mobile station 140 and the nearest oneof the relay stations 130, . . . , 135 is limited, high data ratecommunication can be supported between the mobile station 140 and therelay station 130, . . . , 135 without the use of excessive transmitpower. More background on cellular systems with relay stations can befound in R. Pabst et al., “Relay-based deployment concepts for wirelessand mobile broadband radio,” IEEE Comm. Magazine, vol. 42, no. 9, pp.80-89, 2004. By using directional antennas for the (relatively) longpath between the base station 120 and each of the relay stations 130, .. . , 135 and using omni-directional antennas for the (relatively) shortpath between the relay station 130, . . . , 135 and the mobile station140, capacity and throughput per user is increased as mutualinterference between base stations and relay stations is minimized.

The selection of a relay station has been studied in the past. See, forexample, V. Sreng et al., “Relayer selection strategies in cellularnetworks with peer-to-peer relaying,” IEEE VTC Fall '03, Orlando, Fla.,October 2003. Selection criteria can be the distance, the propagationpath, carrier to noise ratio (C/N) or carrier to interference ratio(C/I), or a combination of any of these. However, in order to carry outthe selection process, a network feature has to be implemented includingmeasurement procedures for supporting the selection process.

In a conventional cellular network, the base stations broadcast acontrol channel (BCCH) or beacon channel. The mobile station can usethis BCCH to obtain information necessary to synchronize itself to thenetwork, to register to the system, and to make and accept calls. Thereceived signal strength indicator (RSSI) and the quality of the BCCHcan be used as parameters in the base station selection procedure. Forproper selection of the relay station, the relay station could broadcasta BCCH as well. However, this would unnecessarily load the system anduse scarce spectrum resources wastefully. Although required only forcall setup and handover support, which are relatively infrequent events,the BCCH would be transmitted continuously by the relay stations.

In addition, BCCH channel planning (frequency or code allocation) forthe relay stations would be an operator's nightmare.

Accordingly, it is desirable to provide an alternative mechanism forsuitably selecting one from a number of relay stations to be used as aconduit for communications between mobile stations and base stations.

SUMMARY

It should be emphasized that the terms “comprises” and “comprising”,when used in this specification, are taken to specify the presence ofstated features, integers, steps or components; but the use of theseterms does not preclude the presence or addition of one or more otherfeatures, integers, steps, components or groups thereof.

In accordance with one aspect of the present invention, the foregoingand other objects are achieved in methods and apparatuses for variouslyoperating mobile stations, relay stations, and base stations in atelecommunications system. In accordance with one aspect, a method ofoperating a mobile station comprises communicating directly with a basestation via a first channel; broadcasting a burst for receipt by aplurality of relay stations; establishing a connection with one of theplurality of relay stations; and communicating indirectly with the basestation via a second channel established with said one of the pluralityof relay stations.

In some embodiments, communication via the first channel proceeds at afirst rate; communication via the second channel proceeds at a secondrate; and the second rate is higher than the first rate.

In another aspect, prior to broadcasting the burst for receipt by theplurality of relay stations, the mobile station requests a higher rateconnection from the base station. The base station can respond byproviding instructions to the mobile station, wherein the instructionsinstruct the mobile station to broadcast the burst for receipt by theplurality of relay stations.

The burst can be transmitted on a channel that is dedicated totransmissions of bursts for receipt by the plurality of relay stations.Alternatively, the burst can be transmitted during a fixed time slotthat is dedicated to transmissions of bursts for receipt by theplurality of relay stations.

In yet another aspect, the first channel can be maintained concurrentlywith the second channel, and the first channel used to communicatecontrol information directly with the base station. In some embodiments,the first channel is used to communicate all control signaling betweenthe mobile station and the base station; and the second channel is usedto communicate only user traffic between the mobile station and the basestation. Alternatively, the first channel can be used to communicateonly control signaling related to mobility and connectionestablishment/release between the mobile station and the base station;and the second channel can be used to communicate user traffic andcontrol signaling associated with the user traffic between the mobilestation and the base station.

In other aspects, operation of a relay station in a telecommunicationssystem comprises scanning for a burst that was broadcast by a mobilestation; receiving the burst; measuring one or more characteristics ofthe received burst that are indicative of a quality of reception; andusing a direct link with a base station in the telecommunications systemto communicate information indicative of the quality of reception.

In another aspect relating to operation of a relay station, aftercommunicating information indicative of the quality of reception to thebase station, the relay station receives instructions from the basestation, wherein the instructions instruct the relay station toestablish a direct connection with the mobile station, wherein the relaystation uses the direct connection with the mobile station to relaycommunications exchanged between the mobile station and the basestation.

Scanning for the burst can comprise, for example, scanning continuouslyon a random access channel for bursts from the mobile station.Alternatively, scanning for the burst can comprise receiving one or moreinstructions from the base station, and in response to the one or moreinstructions, scanning on a random access channel for bursts from themobile station.

In other aspects, operation of a base station in a mobiletelecommunication system comprises communicating directly with a mobilestation via a first channel; receiving a request for a higher rateconnection from the mobile station; instructing the mobile station tobroadcast a burst for receipt by a plurality of relay stations;receiving, from each of one or more relay stations, informationindicative of a quality of reception of the burst from the mobilestation; using the information from each of the one or more relaystations as a basis for selecting one of the one or more relay stations;and communicating indirectly with the mobile station via the selectedrelay station.

In another aspect, before communicating indirectly with the mobilestation via the selected relay station, the base station instructs theselected relay station to establish a direct connection with the mobilestation.

In still another aspect, in response to receiving the request for thehigher rate connection from the mobile station, the base stationinstructs one or more relay stations to scan for a burst to be broadcastby the mobile station.

The one or more relay stations can be, for example, all of the relaystations with which the base station can communicate. Alternatively, theone or more relay stations can be selected from all of the relaystations with which the base station can communicate based on relaystation location relative to a location of the mobile station.

In still another aspect relating to operation of a base station, a basestation communicates indirectly with a mobile station via a first relaystation; instructs the mobile station to broadcast a burst for receiptby a plurality of relay stations; receives, from each of one or morerelay stations, information indicative of a quality of reception of theburst from the mobile station; and uses the information from each of theone or more relay stations as a basis for selecting one of the one ormore relay stations. If the selected relay station is not the firstrelay station, the base station provides the mobile station withinstructions that will cause the mobile station to communicateindirectly with the base station via the selected relay station.

In yet another aspect, the instructions that will cause the mobilestation to communicate indirectly with the base station via the selectedrelay station identify a physical channel to be used by the mobilestation.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the invention will be understood byreading the following detailed description in conjunction with thedrawings in which:

FIG. 1 illustrates an exemplary arrangement in a mobiletelecommunications system comprising a cell 125 with a base station 120,a mobile station 140, and several relay stations 130, . . . , 135.

FIG. 2 is a diagram of a mobile station operating in a cell thatincludes a base station and a number of relay stations.

FIG. 3 is a diagram of a mobile station transmitting random accesspackets in accordance with an aspect of the invention.

FIG. 4 is a diagram of two high rate connections being established—afirst connection between a base station and a relay station, and asecond connection between a relay station and a mobile station.

FIG. 5 is a flow diagram illustrating steps carried out in accordancewith an exemplary embodiment.

DETAILED DESCRIPTION

The various features of the invention will now be described withreference to the figures, in which like parts are identified with thesame reference characters.

The various aspects of the invention will now be described in greaterdetail in connection with a number of exemplary embodiments. Tofacilitate an understanding of the invention, many aspects of theinvention are described in terms of sequences of actions to be performedby elements of a computer system. It will be recognized that in each ofthe embodiments, the various actions could be performed by specializedcircuits (e.g., discrete logic gates interconnected to perform aspecialized function), by program instructions being executed by one ormore processors, or by a combination of both. Moreover, the inventioncan additionally be considered to be embodied entirely within any formof computer readable carrier, such as solid-state memory, magnetic disk,optical disk or carrier wave (such as radio frequency, audio frequencyor optical frequency carrier waves) containing an appropriate set ofcomputer instructions that would cause a processor to carry out thetechniques described herein. Thus, the various aspects of the inventionmay be embodied in many different forms, and all such forms arecontemplated to be within the scope of the invention. For each of thevarious aspects of the invention, any such form of embodiments may bereferred to herein as “logic configured to” perform a described action,or alternatively as “logic that” performs a described action.

In one aspect, a method is presented in which the mobile station plays apivotal role in the relay station selection procedure. A mobile station240 operating in accordance with the invention is depicted in FIG. 2.The system in which the mobile station 240 operates comprises a basestation 220, and several relay stations 230, . . . , 235. Preferably,the mobile station 240 is locked onto the BCCH 210 of the base station220 as schematically illustrated in FIG. 2. The base station 220 createsa macro-cell 225 in which the relay stations 230, . . . , 235 areplaced. It should be noted that the base station 220 itself could be amini-base station or pico-base station in the overall hierarchy of thecellular system. The relay stations 230-235 are locked onto the basestation 220 via a packet-switched connection, for example GPRS.

Using a conventional cellular protocol like GSM, GPRS, or WCDMA, themobile station 240 may request a high data rate channel (e.g., HSDPA inenhanced 3G systems). As mentioned in the Background section above,communication at this high data rate will require a higher minimumreceived power P_(rx) in order to maintain acceptable BER rates.Accordingly, in order to effectively reduce the distance betweentransmitting and receiving antennas for this service, the network willuse a relay station.

Selecting which of the relay stations 230-235 to use for the high datarate channel will be based on some predetermined criterion or criteria.However, instead of the network sending broadcast control signals toallow the mobile station to perform a measurement, the inverse techniqueis applied. Referring to FIG. 3, the mobile station 240 broadcasts arandom access (RA) packet 310. It is intended that some number of thesurrounding relay stations 230, . . . , 235 will receive the RA packetand analyze the quality. The relay stations 230, . . . , 235 musttherefore scan the random access channel. This may be performedcontinuously by the relay stations 230, . . . , 235. The random accesschannel can be defined in any of a number of ways, the particular wayselected not being an essential aspect of the invention. For example,the random access channel can be characterized by a dedicated frequencyin a frequency division multiple access (FDMA) system. Alternatively,the random access channel can be characterized as one or more dedicatedtime slots in a time division multiple access (TDMA) system. In yetanother alternative, the random access channel can be characterized by adedicated spreading code in a spread spectrum system. Yet otheralternatives can be derived by, for example, combining any of thesedescribed channel access techniques. When dedicated time slots are used,the relay stations need to be time aligned with the base station 220,but this is no problem because the relay stations 230, . . . , 235 arealready locked onto the BS 220, as described above.

Alternatively, the scanning by the relay stations 230, . . . , 235 canbe done on demand: the base station 220 knows when the mobile station240 will issue a random access packet 310 and can therefore instruct therelay stations 230, . . . , 235 to scan the random access channel duringa limited time window. For example, the base station 220 can instructall relay stations 230, . . . , 235 to perform a scan when it has nolocation information about the mobile station 240. Alternatively, if thebase station 220 has information about the location of the mobilestation 240, it can instruct only those ones of the relay stations 230,. . . , 235 that are in the neighborhood of the mobile station 240 toperform a scan. For example, in FIG. 3, the base station 220 may haveangular information which informs that the mobile station 240 is in theright half plane of the cell 225. The base station 220 could theninstruct only the relay stations 233, 234 and 235 to scan the randomaccess channel.

Each relay station that receives the random access burst 310 responds bydetermining the quality of the reception and forwarding this informationto the base station 220. The quality information may include, forexample, a signal strength level, a BER indication, soft information, orother signal features. In addition, the information may include a timestamp and an identification number (e.g., a random identificationnumber) that could be included in the random access packet.

Based on the information gathered from different relay stations, thebase station 220 decides which of the relay stations 230, . . . , 235 isoptimal to connect the mobile station 240 to. In FIG. 4, it is assumedthat the signal quality was best for relay station 235. The base station220 then establishes a first high-rate connection 410 to the relaystation 235 (preferably via directional antennas) and a second high-ratechannel 420 is established between the relay station 235 and the mobilestation 240. Establishment of the second high-rate channel 420 can beperformed under the direction of the base station 220. Alternatively, itcan be locally arranged by suitable signaling between the relay station235 and the mobile station 240.

In another aspect, the procedure as described above can also be carriedout during handover. During the high-rate connection (or during a pausein the high-rate communications), the random-access packet 310 is sentby the mobile station 240 and is picked up by surrounding one of therelay stations 230, . . . , 235. The base station 220 may then instructthe current relay station to handover the connection to another relaystation. The instructions may, for example, identify a physical channel(e.g., characterized by frequency, time slot, and/or spreading code,depending on the type of system in use) to be used by the mobilestation.

Preferably, the mobile station 240 remains in communication with thebase station 220 for control signaling. That is, while exchanging userinformation with the base station 220 indirectly via the relay stationusing the second channel 420, the mobile station 240 remains locked tothe base station 220 via the first channel 430. In one embodiment, allcontrol signaling may be carried over the first channel 430. In analternative embodiment, only control signaling related to mobility andconnection establishment/release may be carried by the first channel430, whereas control signaling associated with the user traffic isrelayed via the relay station over the second channel 420.

The mobile station 240 remains locked to the core network of the mobilesystem via the first channel 430. This facilitates handovers not onlybetween relay stations belonging to the current base station 220, butalso between relay stations belonging to different base stations. Inaddition, when the second channel 420 is released, the mobile station240 remains locked to the mobile system and does not have to carry out aconnection setup and/or registration procedure.

FIG. 5 is a flow diagram illustrating steps carried out in accordancewith one of many possible exemplary embodiments. In the figure, stepscarried out by a mobile station are aligned under the heading “MS”;steps carried out by a relay station are aligned under the heading “RS”;and steps carried out by a base station are aligned under the heading“BS”. Also, boxes depicted in dashed lines indicate steps/actions thatmay not be present in all embodiments. (The boxes depicted in dashedlines are to be distinguished from the dashed lines that are intended toindicate links between actions taken by the various entities at theterminal points of the dashed lines.)

To begin this example, it is assumed that a mobile station iscommunicating with the base station by means of a lower rate channel(step 501). The mobile station checks to determine whether a higher ratechannel is needed (decision block 503). If not (“NO” path out ofdecision block 503), then processing returns to communication via thelower rate channel at step 501.

If a higher rate channel is needed (“YES” path out of decision block503), then the mobile station requests a higher rate connection from thebase station (step 505). The base station receives this request (step507) and instructs the mobile station to broadcast a random accesspacket on the random access channel, and also instructs the relaystations (e.g., all relay stations, or alternatively only those relaystations known to be in the vicinity of the mobile station) to scan forthe random access packet to be broadcast by the mobile station (step509).

In response to the base station's instructions, each relay station scansfor a random access packet (step 510). The mobile station also receivesthe base station's instructions (step 511). When the mobile stationbroadcasts its random access packet (step 513), it is received by one ormore relay stations (step 515). A relay station that receives a randomaccess burst from a mobile station performs one or more measurements todetermine the quality of reception (step 517). Information about thequality of reception is then forwarded by the relay station to the basestation (step 519).

The base station collects the reception quality information from the oneor more relay stations that supply it (step 521). Based on thiscollected data, the base station decides which relay station would bethe best choice for use by the mobile station (step 523). The basestation then selects this relay station for use by the mobile stationand instructs that relay station to establish the necessary high rateconnections (step 525).

The selected relay station receives the instructions (step 527) andresponds by establishing a first high rate connection with the basestation (step 529). This may involve some signaling between the selectedrelay station and the base station (step 531).

The selected relay station also exchanges the necessary signals with themobile station to establish a second high rate connection between therelay station and the mobile station (steps 533 and 535). The signalsmay, for example, identify a physical channel (e.g., characterized byfrequency, time slot, and/or spreading code, depending on the type ofsystem in use) to be used by the mobile station.

Having established the first and second higher rate connections, themobile station may then communicate with the base station by means of ahigher rate channel via the relay station.

The various aspects of the invention provide a number of advantages. Forexample, the signaling load associated with selection of a relay stationis reduced compared to conventional techniques. Also, relay stations donot have to be involved in the broadcast scheme of the cellular network.A further advantage is that the quality of reception in the relaystations is determined rather than in the mobile stations. Since therelay stations tend to be placed at strategic positions with regard tocoverage, they are more exposed to interference. Therefore, theselection of the uplink channel is more critical. A selection based onmeasurements in the relay stations will therefore produce a more robustconnection.

The invention has been described with reference to particularembodiments. However, it will be readily apparent to those skilled inthe art that it is possible to embody the invention in specific formsother than those of the embodiment described above. The describedembodiments are merely illustrative and should not be consideredrestrictive in any way. The scope of the invention is given by theappended claims, rather than the preceding description, and allvariations and equivalents which fall within the range of the claims areintended to be embraced therein.

1. A method of operating a mobile station in a telecommunicationssystem, the method comprising: communicating directly with a basestation via a first channel; broadcasting a random access burst forreceipt by a plurality of relay stations; establishing a connection on asecond channel with one of the plurality of relay stations; andcommunicating indirectly with the base station via the second channelestablished with said one of the plurality of relay stations.
 2. Themethod of claim 1, wherein: communication via the first channel proceedsat a first rate; communication via the second channel proceeds at asecond rate; and the second rate is higher than the first rate.
 3. Themethod of claim 2, comprising: prior to broadcasting the random accessburst for receipt by the plurality of relay stations, requesting ahigher rate connection from the base station.
 4. The method of claim 3,comprising: after requesting the higher rate connection from the basestation, receiving instructions from the base station, wherein theinstructions instruct the mobile station to broadcast the random accessburst for receipt by the plurality of relay stations.
 5. The method ofclaim 1, wherein broadcasting the random access burst comprises:transmitting the random access burst on a channel that is dedicated totransmissions of random access bursts for receipt by the plurality ofrelay stations.
 6. The method of claim 1, wherein broadcasting therandom access burst comprises: transmitting the random access burstduring a fixed time slot that is dedicated to transmissions of randomaccess bursts for receipt by the plurality of relay stations.
 7. Themethod of claim 1, comprising: maintaining the first channelconcurrently with the second channel; and using the first channel tocommunicate control information directly with the base station.
 8. Themethod of claim 7, wherein: the first channel is used to communicate allcontrol signaling between the mobile station and the base station; andthe second channel is used to communicate only user traffic between themobile station and the base station.
 9. The method of claim 7, wherein:the first channel is used to communicate only control signaling relatedto mobility and connection establishment/release between the mobilestation and the base station; and the second channel is used tocommunicate user traffic and control signaling associated with the usertraffic between the mobile station and the base station. 10-23.(canceled)
 24. An apparatus for operating a mobile station in atelecommunications system, the apparatus comprising: logic thatcommunicates directly with a base station via a first channel; logicthat broadcasts a random access burst for receipt by a plurality ofrelay stations; logic that establishes a connection on a second channelwith one of the plurality of relay stations; and logic that communicatesindirectly with the base station via the second channel established withsaid one of the plurality of relay stations.
 25. The apparatus of claim24, wherein: communication via the first channel proceeds at a firstrate; communication via the second channel proceeds at a second rate;and the second rate is higher than the first rate.
 26. The apparatus ofclaim 25, comprising: logic that requests a higher rate connection fromthe base station before the random access burst is broadcast for receiptby the plurality of relay stations.
 27. The apparatus of claim 26,comprising: logic that, after the higher rate connection from the basestation has been requested, receives instructions from the base station,wherein the instructions instruct the mobile station to broadcast therandom access burst for receipt by the plurality of relay stations. 28.The apparatus of claim 24, wherein the logic that broadcasts the randomaccess burst comprises: logic that transmits the random access burst ona channel that is dedicated to transmissions of random access bursts forreceipt by the plurality of relay stations.
 29. The apparatus of claim24, wherein the logic that broadcasts the random access burst comprises:logic that transmits the random access burst during a fixed time slotthat is dedicated to transmissions of random access bursts for receiptby the plurality of relay stations.
 30. The apparatus of claim 24,comprising: logic that maintains the first channel concurrently with thesecond channel; and logic that uses the first channel to communicatecontrol information directly with the base station.
 31. The apparatus ofclaim 30, wherein: the logic that uses the first channel to communicatecontrol information directly with the base station uses the firstchannel to communicate all control signaling between the mobile stationand the base station; and the logic that communicates indirectly withthe base station via the second channel uses the second channel tocommunicate only user traffic between the mobile station and the basestation.
 32. The apparatus of claim 30, wherein: the logic that uses thefirst channel to communicate control information directly with the basestation uses the first channel to communicate only control signalingrelated to mobility and connection establishment/release between themobile station and the base station; and the logic that communicatesindirectly with the base station via the second channel uses the secondchannel to communicate user traffic and control signaling associatedwith the user traffic between the mobile station and the base station.33-46. (canceled)